The present invention relates to circuits and methods for controlling the operation of solid state switches, and more particularly to a circuit for reducing the power consumed during operation of a solid state switch by limiting switch operation to periods when the voltage across the switch is less than a predetermined value.
Switches may consume and waste power if the switch is operated when the voltage across the switch is high. Solid state switches require a finite period of time to transition from being highly resistive (off) to being conductive (on), and vice versa. During this period of time, both the voltage across the switch and the current through the switch are changing. As will be appreciated, during the period of time when both voltage and current are present, power is being consumed by the switch. While the power consumed by solid state switches may be reduced by decreasing the time required for the transition, the time cannot be reduced to zero.
In the event a signal across the switch is time varying, power consumption may be reduced by selecting a time for operating the switch when the voltage across the switch is low. For example, the voltage across the switch may be zero during transitions from plus to minus in a sinusoidal (e.g., alternating current) signal.
Telephone systems may include solid state switches for providing an alternating current ring signal to a telephone, and the power consumed by such switches is desirably reduced.
With reference now to FIG. 1, a telephone system 10 (including private-area branch exchanges--PABX) may include a central office (CO) connected to subscriber telephones 12 through a subscriber line interface circuit (SLIC) 14. SLICs perform various functions, one of which is to provide a ring signal to a telephone to indicate that the telephone is being called. A ring signal generator 16 provides a constantly running ac signal (typically 93 volts rms) for ringing the telephone. When a telephone is to be rung, the SLIC 14 operates switches 18 to connect the ring signal to the telephone 12 and disconnect the telephone 12 from the voice signals that will be provided by the SLIC 14.when the call is answered. In newer telephone systems, the switches 18 are typically solid state switches. The SLIC 14 also provides power from a battery 20 (typically 48 volts dc) to operate the system. The battery 20 also provides a dc bias for the ring signal. Further discussion of SLICs for telephone systems may be found in the article "SLIC Ejects Relays From PABX Line Cards" by Frank Goodenough, appearing in the Jul. 11, 1994 issue of Electronic Design, that is incorporated by reference.
With reference now to FIG. 2, the switches 18 may be operated by a signal from a bank of switch drivers (referred to herein as a single driver 22). The driver 22 receives instructions from a gate 24 that provides a command to the driver 22 when a ring command 26 has been received indicating that the telephone is to be rung, provided the ac ring signal from the ring generator 16 is within a predetermined number of volts of a zero crossing. To this end, it is known to connect a comparator 28 to both sides of the ring generator 16 to provide a signal (e.g., a voltage) to the gate 24 that is operated when the ring signal is within a predetermined number of volts of a zero crossing, and the ring command 26 has been received.
This arrangement works reasonably well for systems that have a ring generator 16 and source of dc bias 20 connected such as illustrated in FIGS. 1 and 2. However, not all telephone systems are arranged in this manner. For example, in some systems the source of dc bias 20 is connected between the ring generator 16 and ground, and in other systems a second ring generator is added. Each of these arrangements requires different connections for determining the zero crossing of the ring signal, and it is desirable to provide a circuit for controlling the operation of the switches 18 that is operable regardless of the manner in which the ring generator and source of dc bias are connected.
Another problem with the switch controlling circuits of the prior art is that they use discrete components, primarily due to the high voltages that are being carried. Discrete components add cost and require more space than components that are in integrated circuits. It is desirable to provide an integrated circuit for controlling the switches in order to decrease circuit size and cost and to increase reliability. Integrated circuits. also provide the added benefit of reducing unwanted transient currents and noise.
Accordingly, it is an object of the present invention to provide a novel circuit and method for controlling a switch that obviates the problems of the prior art.
It is another object of the present invention to provide a novel circuit and method for controlling a solid state switch in which the voltage across the switch is sensed to determine when the switch may be operated.
It is yet another object of the present invention to provide a novel circuit and method for controlling switches that connect a ring signal to a telephone in which the voltage across the switches and the telephone is evaluated to determine when the switches may be operated.
It is still another object of the present invention to provide a novel circuit and method for controlling switches that connect a ring signal to a telephone in which the voltages at the switch terminals are reduced before comparison so that the circuit may be embodied in an integrated circuit.
It is a further object of the present invention to provide a novel circuit and method for controlling solid state switches that connect a ring signal to a telephone in which the circuit for controlling the switches and the switches are in an integrated circuit.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.